A vacuum countergravity casting process using a gas permeable mold is described in such prior art patents as the Chandley et al U.S. Pat. No. 4,340,108 issued July 20, 1982 and U.S. Pat. No. 4,606,396 issued Aug. 19, 1986. That countergravity casting process is of the mold-immersion type and involves providing a mold having a porous, gas permeable upper mold member (cope) and a lower mold member (drag) secured together, sealing a vacuum chamber to the mold such that the vacuum chamber confronts the gas permeable upper mold member, submerging the bottom side of the lower mold member in an underlying molten metal pool and evacuating the chamber to draw molten metal through one or more ingate passages in the lower mold member and into one or more mold cavities formed between the upper and lower mold members. The mold used in that vacuum countergravity casting process typically includes a rigid, self-supporting, resinbonded upper mold member and lower mold member secured together by suitable means.
Another casting process, known in the art as the "lost foam" process, involves pouring molten metal into a foamed plastic pattern surrounded by a porous, unbonded (binder free) sand mold such that the molten metal destroys (vaporizes) the pattern and replaces it in the sand before the sand collapses. The solidified metal thus assumes the shape of the foamed plastic pattern and the pattern destruction products escape into the sand. The lost foam process has been proposed for use in conjunction with both gravity and countergravity poured metal as exemplified by Wittmoser U.S. Pat. No. 4,085,790 issued Apr. 25, 1987 and Denis U.S. Pat. No. 4,616,689 issued Oct. 14, 1986, respectively.
It is an object of the present invention to provide an improved, economical countergravity casting process of the mold-immersion type which essentially eliminates the need for costly mold-making particulate (e.g., resin-containing sand) and separate mold-making and mold-handling equipment as well as significantly reduces the time required to carry out the process.
It is another object of the present invention to provide an improved, economical countergravity casting process and apparatus wherein an inherently unstable mass of particulate mold material is held in an open bottom container around a destructible pattern therein by an external fluid pressure exerted on the bottom side of the mass in excess of internal pressure in the container and wherein the bottom side of the particulate mass is submerged in an underlying molten metal pool in such a way as to permit drawing molten metal to the pattern to destroy and replace it in the particulate mass.